Method of fabricating an MTJ with low areal resistance

ABSTRACT

A method of fabricating a magnetoresistive tunnel junction including forming a thin, continuous layer of aluminum alloy on the surface of a first magnetic layer, the continuous layer of aluminum alloy including greater than 90% aluminum and traces of materials having atoms that are different from the atoms of the aluminum to produce grains which are smaller than grains of pure aluminum. The continuous layer of aluminum alloy is oxidized, nitridized, or both to produce a continuous layer of non-conductive material and a second magnetic layer is formed on the layer of non-conductive material.

FIELD OF THE INVENTION

The present invention pertains to methods of fabricatingmagnetoresistive tunneling junctions for memory cells and morespecifically to methods of manufacturing magnetoresistive tunnelingjunctions with low areal resistance.

BACKGROUND OF THE INVENTION

A magnetic random access memory (MRAM) is a non-volatile memory whichbasically includes a giant magnetoresistive (GMR) material or magnetictunneling junction (MTJ) structure, a sense line, and a word line. TheMRAM employs the magnetic vectors to store memory states. Magneticvectors in one or all of the layers of GMR material or MTJ are switchedvery quickly from one direction to an opposite direction when a magneticfield is applied to the GMR material or MTJ over a certain threshold.According to the direction of the magnetic vectors in the GMR materialor MTJ, states are stored, for example, one direction can be defined asa logic "0", and another direction can be defined as a logic "1". TheGMR material or MTJ maintains these states even without a magnetic fieldbeing applied. The states stored in the GMR material or MTJ can be readby passing a sense current through the cell in a sense line because ofthe difference between the resistances of the two states.

Magnetic tunneling junction (MTJ) structure or cells include at least apair of magnetic layers with a non-magnetic, non-conducting tunnel layersandwiched therebetween. When a sense voltage is applied between thepair of magnetic layers, electrical carriers travel between the pair ofmagnetic layers by tunneling through the non-magnetic, non-conductingtunnel layer sandwiched between the magnetic layers. The resistance tothe sense current is a maximum when the magnetic vectors of the pair ofmagnetic layers are anti-parallel and minimum when the magnetic vectorsof the pair of magnetic layers are parallel. The difference between themaximum and minimum resistance is commonly referred to as themagnetoresistance (MR) ratio.

Further, the minimum resistance of the MTJ cell (commonly referred to asthe areal resistance) is determined by the construction and materials ofthe MTJ cell. Clearly, in an ideal MTJ cell the areal resistance wouldbe very low or zero and the maximum resistance would be very high orinfinite, similar to an ideal switch. Prior art attempts to reduce theareal resistance include depositing a layer of pure aluminum on thelower magnetic layer and then oxidizing the aluminum layer in oxygenplasma. A problem with this procedure is that as the aluminum layer isdeposited, pinholes tend to form, especially if the layer is thin. Asthe aluminum is oxidized, some of the pinholes tend to remain andproduce shorts in the MTJ cell when the second magnetic layer isdeposited on the aluminum oxide layer. To overcome the pinhole problem,one possible solution is to deposit the aluminum layer at lowtemperatures (e.g. the temperature of liquid nitrogen) for reducing thesize of the grains. Some of the problems with this method are that itinvolves extensive heating and cooling cycles, takes a long time, costsmore and hence is not a method which can be used in manufacturing.

Accordingly, it would be highly advantageous if MTJ cells could befabricated at room temperature without the problem of pinholes and thelike.

It is a purpose of the present invention to provide a new and improvedmethod of fabricating MTJ cells with reduced areal resistance.

It is another purpose of the present invention to provide a new andimproved method of fabricating MTJ cells with high quality barriers ortunnel layers.

It is a still another purpose of the present invention to provide a newand improved method of fabricating MTJ cells with high magnetoresistanceratios.

It is a further purpose of the present invention to provide a new andimproved method of fabricating MTJ cells with thinner and continuousmetal layers for the formation of the barrier or tunnel layer.

It is still a further purpose of the present invention to provide a newand improved method of fabricating MTJ cells which does not requireextensive cooling and heating cycles and which is easily adaptable tomanufacturing.

SUMMARY OF THE INVENTION

The above problems and others are at least partially solved and theabove purposes and others are realized in a method of fabricating amagnetoresistive tunnel junction including forming a continuous layer ofmaterial on the surface of a first magnetic layer, the layer of materialincluding a dominant material and traces of materials having atoms thatare different from the atoms of the dominant material to produce grainswhich are smaller than grains of the dominant material alone. Thecontinuous layer of material is then oxidized, nitridized, or somecombination of the two to produce a layer of non-conductive material anda second magnetic layer is formed on the layer of non-conductivematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIG. 1 through FIG. 4 are simplified and enlarged sectional viewsillustrating various intermediate structures in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings in which like characters are designated withsimilar numbers throughout, FIG. 1 illustrates a structure including asupporting substrate 10 with a magnetic layer 11 formed thereon. Whilesupporting substrate 10 is illustrated as a single layer for simplicity,it will be understood by those skilled in the art that it may includeadditional layers for various purposes, such as buffer layers,insulating or conducting layers, etc., and all such layers areconsidered as included in supporting substrate 10. Magnetic layer 11 hasan upper surface 12 which is formed as smooth as possible for thedeposition of further layers thereon. Magnetic layer 11 is formed of anyof the various materials, or multiple layers of materials, well known inthe art and will not be described in great detail herein.

While the magnetic cell or cells described herein are illustrated insectional views for convenience, it should be understood that thefigures represent a portion of a cell or an array of cells and each cellcan be rectangularly shaped or circular, square or diamond shaped, orelliptical, if desired. As is known in the art, cells that arerectangularly shaped have magnetization vectors that are positionedsubstantially along the length of the cell and maintained parallel tothe length by the physical anisotropy. To achieve this, the width of thecell is formed to be smaller than the width of the magnetic domain wallsor transition width within the magnetic layers. Typically, widths ofless than 1.0 to 1.2 microns result in such a constraint. Generally, toachieve high density the width is less than one micron and is as smallas can be made by manufacturing technology, and the length is greaterthan the width. Also, thicknesses of the magnetic layers, e.g. layer 11,are approximately three to ten nanometers and may be different for eachmagnetic layer in some embodiments. The difference in thicknesses affectthe switching points and are utilized in some structures for reading andwriting cells.

When the aspect ratio (length to width) of a single magnetic layer isclose to one, such as for circular, square or diamond shaped, orelliptical shaped cells, the switching field from shape anisotropy isminimum. In the case of circularly shaped cells, for example, thepreferred magnetization direction is determined by uniaxial crystalfield anisotropy (or magnetic crystalline anisotropy). This preferredmagnetization direction is set during film deposition by a bias field orby annealing the film after deposition in a high magnetic field (e.g.several kOe) at elevated temperatures (e.g. 200° C. to 300° C.). In thecase of a square or diamond shape, for example, the uniaxial crystalanisotropy is set along a diagonal direction of the square. In the caseof an elliptically shaped cell, the uniaxial crystal anisotropy is setalong the long axis of the cell. The main idea is to minimize the shapeeffect, which contributes to the rise in required switching fields atnarrow cell widths, and to utilize crystal field anisotropy to set thepreferred magnetization direction needed by a memory cell.

Referring now to FIG. 2, a continuous layer 15 of metal is formed onsurface 12 of magnetic layer 11. For purposes of this disclosure theterm "continuous" refers to a layer of material which is approximatelycoextensive with magnetic layer 11 and which does not include anypinholes or the like. Layer 15 includes a dominant material or element,which can be easily oxidized, nitridized, or some combination of thetwo, and traces of one or more other materials or elements having atomsthat are different from the atoms of the dominant metal. Generally, theamount of the dominant element will be greater than 90% including onlyan amount sufficient to produce grains in layer 15 which are smallerthan grains of the dominant element alone so as to produce a continuouslayer. In a preferred embodiment, the dominant element is aluminum andthe trace materials can be any or all of Cu, Si, Ta, Ti, or the like.

In one specific technique a dominant metal and a trace metal or metalsare deposited simultaneously so as to arrive at a mixture that resultsin the smaller grains and, thus, the continuous layer. In a secondtechnique, seed material is deposited on surface 12 of magnetic layer 11by any convenient method. The seed material may or may not be acontinuous layer. The dominant material is then formed with the use ofthe seed material, the presence of the seed material insuring asubstantially thinner continuous layer of the dominant material. In apreferred embodiment, the seed material includes one or more of Cu, Si,Ta, Ti, or the like and the dominant material includes aluminum.

In either of the above techniques, layer 15 is continuous with nopinholes or the like and with a thickness in a range of approximately0.3 nm to 3 nm and preferably approximately 1.5 nm. It has been found,for example, that the minimum thickness of a continuous aluminum layer15 for producing a quality barrier layer is approximately 1.5 nm, withthinner continuous aluminum layers resulting in a reduced MR ratio.However, in instances where a dominant material other than aluminum isused, the thinnest continuous layer possible without a reduction in theMR ratio may be different. The addition of a trace material in aluminumcan also lower its tunneling energy barrier and, hence, lower theresistance of the MTJ.

Once continuous layer 15 is in place, a treating step is performed toproduce a barrier or tunnel layer 16, as illustrated in FIG. 3, oftreated non-conductive material. The treatment may include, for example,plasma oxidation, nitridation, or both. Generally, continuous layer 15can be made very thin while still maintaining its integrity, whichresults in a very thin barrier or tunnel layer 16 and, consequently, asubstantially reduced areal resistance. Further, the fact that layer 15is continuous substantially improves the quality and reliability ofbarrier or tunnel layer 16.

Referring to FIG. 4, a magnetic layer 18 is deposited over barrier ortunnel layer 16 and any electrical connections, passivation, etc. (notshown) are performed to provide a complete cell 20 and/or an array ofcells. Generally, the thicknesses of magnetic layer 18 is approximatelythree to ten nanometers and may be different than or the same asmagnetic layer 11 in different embodiments. As explained above, thedifference in thicknesses of the magnetic layers affect the switchingpoints and are utilized in some structures for the functions of readingand writing the cells.

In a specific example, layer 11 is formed of cobalt (Co) approximately100 Å thick (generally in a range of 10 Å to 200 Å), layer 16 is formedof aluminum oxide (Al₂ O₃) approximately 25 Å thick (generally in arange of 10 Å to 100 Å), and magnetic layer 18 is formed of nickel iron(NiFe) approximately 100 Å thick (generally in a range of 10 Å to 200Å). The change of resistance versus the resistance (ΔR/R) is generallyin a range of 10% to 30%. Thus, the state of magnetic cell 20 isrelatively easily sensed by passing a sense current therethrough frommagnetic layer 11 to magnetic layer 18 (or vice versa). The change ofresistance in cell 20 is easily read as a change in voltage drop acrosscell 20 which can conveniently be used in conjunction with memory arraysand the like.

Thus, a new and improved method of fabricating MTJ cells with reducedareal resistance has been disclosed. The new and improved methodprovides the fabrication of MTJ cells with high quality barriers ortunnel layers and with high magnetoresistance ratios. The new andimproved method provides for the fabrication of thinner and continuousmetal layers for the formation of the barrier or tunnel layer in MTJcells is easily adaptable to manufacturing processes.

While we have shown and described specific embodiments of the presentinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular forms shown and we intend inthe appended claims to cover all modifications that do not depart fromthe spirit and scope of this invention.

What is claimed is:
 1. A method of fabricating a magnetoresistive tunneljunction comprising the steps of:providing a first magnetic layer ofmaterial with a surface; forming a continuous layer of material on thesurface of the first magnetic layer, the layer of material including adominant material having atoms and traces of materials having atoms thatare different from the atoms of the dominant material; treating thecontinuous layer of material to produce a layer consisting of treatednon-conductive material; and forming a second magnetic layer on thelayer of treated non-conductive material.
 2. A method of fabricating amagnetoresistive tunnel junction as claimed in claim 1 wherein the stepof forming the continuous layer of material including the dominantmaterial includes forming a continuous layer of aluminum.
 3. A method offabricating a magnetoresistive tunnel junction as claimed in claim 2wherein the step of forming the continuous layer of material includingtraces of materials includes forming a continuous layer of aluminum withtraces of one of Cu, Si, Ta, or Ti.
 4. A method of fabricating amagnetoresistive tunnel junction as claimed in claim 1 wherein the stepof forming the continuous layer of material includes forming the layerwith a thickness in a range of 0.3 nm to 3 nm.
 5. A method offabricating a magnetoresistive tunnel junction as claimed in claim 4wherein the step of forming the continuous layer of material includesforming the layer with a thickness of approximately 1.5 nm.
 6. A methodof fabricating a magnetoresistive tunnel junction as claimed in claim 1wherein the step of forming the continuous layer of material includingthe dominant material having atoms and traces of materials having atomsthat are different from the atoms of the dominant material includes thesteps of depositing on the surface of the first magnetic layer a seedmaterial having atoms that are different from the atoms of the dominantmaterial and depositing the dominant material on the seed material.
 7. Amethod of fabricating a magnetoresistive tunnel junction as claimed inclaim 1 wherein the step of forming the continuous layer of materialincluding the dominant material includes forming the continuous layerwith greater than 90% dominant material.
 8. A method of fabricating amagnetoresistive tunnel junction as claimed in claim 1 wherein the stepof treating the continuous layer of material includes one of oxidizing,nitridizing, or a combination thereof.
 9. A method of fabricating amagnetoresistive tunnel junction comprising the steps of:providing afirst magnetic layer of material with a surface; forming a continuouslayer of aluminum alloy on the surface of the first magnetic layer witha thickness in a range of 1 nm to 3 nm, the layer of aluminum alloyincluding greater than 90% aluminum having atoms and traces of materialshaving atoms that are different from the atoms of the aluminum toproduce grains which are smaller than grains of the aluminum alone;oxidizing the continuous layer of aluminum alloy to produce a layer ofoxidized aluminum alloy; and forming a second magnetic layer on thelayer of oxidized aluminum alloy.
 10. A method of fabricating amagnetoresistive tunnel junction as claimed in claim 9 wherein the stepof forming the continuous layer of metal including traces of materialsincludes forming a continuous layer of aluminum with traces of one ofCu, Si, Ta, or Ti.
 11. A method of fabricating a magnetoresistive tunneljunction as claimed in claim 9 wherein the step of forming thecontinuous layer of metal includes forming the layer with a thickness ofapproximately 1.5 nm.
 12. A method of fabricating a magnetoresistivetunnel junction as claimed in claim 9 wherein the step of forming thecontinuous layer of metal including the dominant oxidizable metal havingatoms and traces of materials having atoms that are different from theatoms of the oxidizable metal includes the steps of depositing on thesurface of the first magnetic layer a seed material having atoms thatare different from the atoms of the oxidizable metal and depositing thedominant oxidizable metal on the seed material.